1. Field of the Invention
The present invention relates to a modem apparatus which is provided between a telephone circuit and a terminal apparatus, and, in particular, to a modem apparatus in which direct-current insulation is performed between the telephone circuit and the terminal apparatus.
2. Descriptions of the Related Art
A modem apparatus for an analog circuit is provided between a telephone circuit and a terminal apparatus. A signal supplied from the terminal apparatus is converted into a signal which is suitable for signal transmission through the telephone circuit, and the thus-obtained signal is sent to the telephone circuit. A signal supplied from the telephone circuit is converted into a digital signal which can be easily processed by the terminal apparatus, and the thus-obtained signal is sent to the terminal apparatus.
Generally, in such a modem apparatus, direct-current insulation should be provided between the telephone circuit and the terminal apparatus so that a transmission circuit network is prevented from being damaged. A JATE (Japan Approvals Institute for Telecommunication Equipment) Standard prescribes that `the withstand voltage should be equal to or higher than 250 V` as a Japanese standard. In the United States of America, FCC (Federal Communication Commission) 68 standard prescribes that `the withstand voltage should be equal to or more than 1500 V`.
In order to fulfill these standards, such a modem apparatus normally includes a `circuit transformer` for withstanding the above-mentioned withstand voltage.
FIG. 1 shows an example of such a modem apparatus in the related art in which the direct-current insulation is performed by such a circuit transformer. A signal supplied from a personal computer 1 (the terminal apparatus) is transmitted to the telephone circuit 5 via the modem apparatus 2. The modem apparatus 2 includes a modem circuit 4, a circuit transformer 3, a relay and a photocoupler. The circuit transformer 3 is used for providing a necessary direct-current insulation. The modem apparatus includes a hook-off/on control circuit. (Although a hook-off signal itself is generated by the personal computer 1, the modem apparatus 2 processes the signal.) The modem apparatus 2 controls the connection with the telephone circuit 5 by controlling the relay in accordance with the hook-off signal. The modem apparatus 2 further includes a ringer-signal detecting circuit which detects a ringer signal through the photocoupler and the thus-detected signal is transferred to the personal computer 1.
As a method for enabling miniaturization of such a modem apparatus, the applicant of the present application disclosed a method in Japanese Laid-Open Patent Application No.5-268372 utilizing high-withstand-voltage capacitors (capacitors which withstand a high voltage). FIG. 2 shows an example in which the direct-current insulation is performed by the high-withstand-voltage capacitors. A signal supplied from the personal computer 1 is transmitted to the telephone circuit 5 via the modem apparatus 2. The modem apparatus 2 includes the modem circuit 4, the high-withstand-voltage capacitors 10, 11, and operational amplifiers 12 through 14. In the modem apparatus 2, the capacitors 10, 11 are used for providing the necessary direct-current insulation.
The modem apparatus 2 shown in FIG. 1 includes the circuit transformer 3 which includes cores and coils. As a result, it is not possible to incorporate the modem apparatus 2 into an LSI (Large Scale Integration) circuit. Thus, there is a limit to the miniaturization of the modem apparatus 2.
The modem apparatus 2 shown in FIG. 2 does not need such a circuit transformer, and greatly contributes to thinning of the modem apparatus of PCMCIA (Personal Computer Memory Card International Association) or the like. However, when the modem circuit is used for the telephone circuit, it is necessary to transmit a sound signal of a low frequency equal to or lower than 300 Hz. Therefore, a capacitor having a large capacitance of several .mu.F is needed. As a result, there is a limit to the miniaturization of the modem apparatus, and, it is difficult to incorporate such a modem apparatus into one package.